Surface plasmon resonance sensors and method for detecting samples using the same

ABSTRACT

Disclosed is an optical sensing device including a source unit providing a beam of light with continuously modulated phase retardation between p- and s-polarization components of the light by employing a LCM; a reference unit receiving a first part of the light to provide a reference signal; a SPR sensing unit receiving a second part of the light to induce a phase retardation change between the p- and s-polarization components due to SPR associated with a sample; a probe unit receiving the light after SPR to provide a probe signal; and a detection unit connected to the reference unit and the probe unit to detect characteristics of the sample by comparing the reference signal with the probe signal. By using active phase modulation technologies and differential phase measurement, it is possible to fulfill chemical and biological detection.

TECHNICAL FIELD OF THE INVENTION

This invention relates to an optical sensing device for chemical andbiological detection, and more particularly to a surface plasmonresonance (SPR) sensor which provides a simple and accurate SPR phasemeasurement by making use of a birefringence of a liquid crystalmodulator (LCM) to continuously modulate a phase difference betweens-polarization and p-polarization.

BACKGROUND OF THE INVENTION

SPR sensors have been widely used in a variety of disciplines such aschemical, biochemical, biological, biomedical analysis, pollutionmonitoring, and process control.

SPR is the result of optical excitation of a surface plasmon wave (SPW)along an interface between a conducting material and a non-conductingmaterial. A common technique for their creation is to direct a beam ofelectromagnetic radiation into a glass prism with an angle of incidenceabove the critical angle so that it undergoes total internal reflection.The internal reflection creates an evanescent electromagnetic wave at aregion outside of the prism adjacent to the surface. When a thinconductive film such as gold or silver is deposited on the surface ofthe prism, surface plasmons will be formed.

Various types of optical sensors relying upon SPR measurement have beenreported. These sensing techniques are primarily concerned withanalyzing the angle, wavelength or phase properties of the reflectedbeam to extract the SPR information (Sensors and Actuators B, 54, 3-15,1999). There are two most popular sensing schemes. One is the angularinterrogation scheme which involves a monochromatic light source andmeasuring the intensity variation of the reflected beam at a range. Theother is the wavelength interrogation scheme which uses a broadbandlight source and obtains SPR information by observing the spectralintensity variation at a fixed illumination angle.

In fact, SPR affects not only the intensity of the reflected light beambut also its optical phase at the same time. Researchers including ushave found that the phase response has a steep slope near the SPRabsorption dip (Optical Communication, 150, 5-8, 1998). Based on thisproperty, phase interrogation has been estimated to provide extremelyhigh sensing accuracy.

The first practical SPR phase measurement system was based on heterodyneinterferometry (Sensors and Actuators B, 35-36, 187-191, 1996). It usedan acousto-optical modulator (AOM) to modulate the signal in highfrequency at 140 MHz. In order to obtain the phase information, a localoscillator was employed to shift the AOM modulation frequency at 10 kHzso that a phase meter may be employed to measure the phase differencebetween the reference and the probe signals. The paper describes thatthe estimated sensitivity, because of SPR phase measurement, has threetimes improvement compared to the conventional scheme. Although thissensing scheme can extract the SPR phase information from the reflectedbeam, the design is rather complicated both in the optical andelectronic sections. In the optical part, it requires very preciseoptical alignment when the two optical beams are recombined again toensure formation of detectable interference fringes. In the electronics,there are also many high frequency mixes for processing the signal. Inaddition, the need of acousto-optical modulator (AOM) also inerrablyincreases system complexity as well as costs.

Later Guo et. al. (Applied Optics, 37, 1747-1751, 1998) demonstrated amuch simplified heterodyne phase sensing system using afrequency-stabilized Zeeman laser. In their system the self-frequencyshift between the s- and p-polarizations due to the Zeeman's effect isemployed so that the s- and p-polarizations may interfere with eachother. Thus, a beat signal, which appears at the photodetector, providesthe phase quantity associated with the SPR effect. The beat frequencysignal ranges between tens of kilohertz to several mega-hertz. Such ahigh frequency is too fast to image analysis except using expensivehigh-speed CCD cameras. Therefore, this technique may only findapplications in single sensor instruments.

More recently, a static Mach-Zehnder interferometer has been used byNikitin et. al. (Sensors and Actuators B, 85, 189-193, 2000), forperforming two-dimension SPR phase imaging. The main drawback of thisdesign is that the system is very sensitive to mechanical movements inthe optical components. Small mechanical vibrations in the mirrors orvariations of temperature will inevitably cause the optical beam to moveand thus leading to phase measurement error (Review of Scientificinstruments, 73, 3534-3539, 2002).

It has been reported that ellipsometric measurement (Sensors andActuators B, 51, 331-339, 1998) can also provide SPR phase information.But the drawback is that this technique involves rather cumbersomeprocedures. Ellipsometry equipments, whether using white light or alaser beam, are very slow machines in which several mechanicalcomponents including the polarization analyzer, the wavelengthspectrometer and the goniometers are required to change positionmechanically in order to obtain information. For SPR applications, whichusually require real-time signal reporting, the slow speed fromcommercial ellipsometers is a major disadvantage.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a simple device which canperform accurate SPR phase measurement on a two-dimensional level so asto analyze biological, biochemical, or chemical characteristics of asample.

To achieve the above object, the present invention discloses a methodand a SPR sensing device for chemical and biological detection byemploying an active phase modulation through a liquid crystal modulator(LCM) and a differential phase measurement technique.

According to an aspect of the present invention, an optical sensingdevice comprises a source unit which includes a source emitting a beamof light containing p- and s-polarization components and a phasemodulator modulating a phase retardation between the p- ands-polarization components; a reference unit for receiving and detectinga first part of the light to provide a reference signal indicating themodulated phase retardation; a SPR sensing unit for receiving a secondpart of the light to induce a phase retardation change between the p-and s-polarization components due to SPR associated with a sample; aprobe unit for receiving the light passing the SPR sensing unit toprovide a probe signal indicating the phase retardation change inducedby the SPR sensing unit; and a detection unit, coupled between thereference unit and the probe unit to detect biological, biochemical, orchemical characteristics of the sample by comparing the reference signalwith the probe signal.

According to an embodiment of the present invention, the phase modulatormay be an LCM. The orientation of the liquid crystal can be controlledby a drive voltage of the LCM, so that the phase retardation between thep- and s-polarization components can be continuously modulated byvarying the drive voltage of LCM.

According to another embodiment of the present invention, the sourceunit may further comprise a polarizer for setting an intensity ratiobetween the p- and s-polarization components of the light emitted fromthe source to achieve a high signal-to-noise ratio.

According to another aspect of the present invention, a method fordetecting biological, biochemical, or chemical characteristics of asample comprises: transmitting a beam of light containing p-polarizationand s-polarization components; modulating a phase retardation betweenthe p-polarization and s-polarization components; providing a referencesignal indicating the phase retardation by receiving a first part of thelight; receiving a second part of the light to induce a phaseretardation change between the p-polarization and s-polarizationcomponents due to SPR associated with a sample; providing a probe signalindicating the phase retardation change by receiving the light after SPRassociated with the sample; and detecting biological, biochemical, orchemical characteristics of the sample by comparing the probe signalwith the reference signal.

As stated above, the reference unit is used for providing information onphase retardation introduced by LCM, while the probe unit provides theinformation of phase retardation change due to SPR in addition to thatinduced by LCM. Direct subtraction between the phase values obtainedfrom the probe unit and the reference unit leads to the measurement ofSPR phase change. The LCM plays an important role in the invention,because it can continuously modulate the phase retardation, whichenables self-interference between the s- and p-polarization so thatsignals (e.g., images) captured before and after the beam passing theSPR sensor may be transformed into SPR differential phase signals.Therefore, it is possible to perform accurate SPR phase measurement on atwo-dimensional level so that any local variations of phase retardationchange due to the SPR sensor may be detected. We now have a single beaminterferometer (i.e. no need of a separate reference beam) with inherentnoise immunity. The optical setup becomes extremely simple.

The key features of this invention are: i) using a differential phasemeasurement technique to improve sensing accuracy by eliminating allcommon-mode phase noise, that is, the measurement only corresponding tothe phase change induced by the SPR effect; ii) using an active phasemodulation technique through an LCM to actively modulate the light beamwhich thereby greatly reduces system hardware complexity.

The SPR sensor of the present invention provides many advantages. Firstit offers higher sensitivity measurement in refractive index changesnear on the sensing surface. Second, the stability of phase measurementis enhanced by using differential phase measurement technique, whichreduces disturbance from the environment. Third, the invention can beused in a range of different applications where real time SPR phaseimaging measurement is needed. Fourth, the system design is simple andlow cost both in its optical and electronic parts. Fifth, the inventionmay be easily adapted for use in different types of SPR couplingschemes. Sixth, its small size and low power consumption make it wellsuited for compact and portable system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a SPR sensor according to the presentinvention;

FIG. 2 is a schematic view of an optical light source unit and areference unit illustrated in FIG. 1;

FIG. 3 is a schematic view showing principles of a liquid crystalmodulator illustrated in FIG. 1;

FIG. 4 a shows a structure of an embodiment of a liquid crystalmodulator of the liquid crystal modulator illustrated in FIG. 2;

FIG. 4 b shows a structure of another embodiment of a liquid crystalmodulator of the liquid crystal modulator illustrated in FIG. 2;

FIG. 4 c shows a structure of a further embodiment of a liquid crystalmodulator of the liquid crystal modulator illustrated in FIG. 2;

FIG. 5 shows the relationship between a phase retardation modulation anda drive voltage of a liquid crystal modulator;

FIG. 6 a and FIG. 6 b show the modulation efficiencies of single passand multi-pass enhance phase retardation methods;

FIG. 7 shows the prism coupling scheme according to the presentinvention;

FIG. 8 is a schematic view of a SPR sensor according to the presentinvention;

FIG. 9 shows typical collected waveforms from the probe and referenceunits;

FIG. 10 shows SPR phase measurement obtained from glycerin and watermixtures of various concentrations: 0%, 0.05%, 0.1%, 0.25%, 0.5%, 1%,2%, 4%, 8% and 16% in weight percentage; and

FIG. 11 is a graph showing system stability measured over an hour.

DETAILED DESCRIPTION OF THE INVENTION

The present invention and various advantages thereof will be describedwith reference to exemplary embodiments in conjunction with thedrawings.

FIG. 1 illustrates an embodiment of a phase SPR sensor in accordancewith the present invention. In the embodiment, the phase SPR sensorcomprises: an optical light source unit 100 for providing a beam oflight containing p- and s-polarization components with a modulated phaseretardation between the p- and s-polarization components; a referenceunit 200 for receiving one portion of the beam to provide a referencesignal indicating the modulated phase retardation; a sensing unit 300for receiving another portion of the beam to induce a phase retardationchange between the p- and s-polarization components due to SPRassociated with a sample; a probe unit 400 for receiving the lightpassing the SPR sensing unit so as to provide a probe signal indicatingthe phase retardation change induced by the SPR sensing unit; and adetection unit 500 connected to the reference unit and the probe unit todetect characteristics of the sample by comparing the reference signalwith the probe signal.

As shown in FIG. 2, the optical light source unit 100 comprises anelectromagnetic radiation source 101, a polarizer 102, a phase modulator103, and a beam splitter 104.

The electromagnetic radiation source 101 may comprise a gas laser, asolid state laser, a laser diode, a light emitting diode (LED), a broadband white light source or any other suitable electromagnetic radiationsource. Preferably, a linear polarization He—Ne laser with opticaloutput power of 12 mW is employed in the embodiment. The radiationsource 101 is required to emit a beam of light containing s- andp-polarization components and the polarization of the output light beamis preferably set at 45° off the p-polarization.

The polarizer 102 is provided to select an intensity ratio between thep- and s-polarizations by turning a polarization angle of the polarizer102. The polarization angle of the polarizer 102 is set at 45° off top-polarization optical axis to obtain an equal intensity in both p- ands-polarizations.

The phase modulator 103 is a liquid crystal modulator (LCM) formodulating the optical phase retardation between the p- ands-polarization components. The fast axis of the LCM 103 may be matchedwith the optical axis of p- or s-polarizations to increase a modulationdepth. The modulation principle is described as shown in FIG. 3. The LCM103 receives a light 1300 passing through the polarizer 102. Propagationspeeds of p-polarization component 1301 and s-polarization component1302 of the light 1300 within the LCM 103 depend on the orientation ofthe liquid crystal inside the LCM 103. In here, the orientation of theliquid crystal can be controlled by a drive voltage of the LCM 103. Thatis, the propagation speeds of the p- and s-polarization components 1301and 1302 can be controlled by the drive voltage. For example, at thebeginning of the LCM 103, the phase retardation between the p- ands-polarization components 1301 and 1302 is Δ=0. During the propagationof the p- and s-polarization components 1301 and 1302 within the LCM103, the phase retardation therebetween is increased from Δ to Δ1. And,after passing through the LCM 103, a modulated phase retardation Δ2 isinduced to the p- and s-polarization components 1301′ and 1302′.Moreover, the modulated phase retardation Δ2 can be continuously changedby varying the drive voltage of LCM 103 in steps. According to theembodiment, the drive voltage of the LCM 103 is adjustable ranging from0 to 13V and the frequency of the LCM 103 is set to be 10 Hz.

A relationship between the phase retardation modulation and the drivevoltage of the LCM 103 is measured and shown in FIG. 5. It is understoodthat the relationship of the phase retardation between p- ands-polarizations versus the input drive voltage is not a linearrelationship. This non-linear characteristic may be easily convertedback to a linear function via a pre-determined data transformationsoftware program. Traditional SPR phase changes observed from the samplemay be as high as 2π. The embodiment is therefore required to performphase retardation with the range from 0 to 2π. Since a phase retardationof π and 3π can generate identical interference fringe patterns, theonly way to remove this ambiguity is to continuously keep track of theamount of the phase retardation being introduced to the light. Also, inorder to precisely detect the phase retardation, the modulation depth ofthe phase retardation should be many cycles to 2π. This is because finalsignal waveforms transformed using the pre-determined function will be atruncated sine wave, from which Fast Fourier Transform will producerequired differential phase detection. One can improve detectionprecision if we increase the modulation depth, i.e., the number ofcycles covered by the truncated since wave. We have proposed somesolutions to resolve this problem in the following simple ways.

In order to achieve a sufficient modulation depth of the phaseretardation, the LCM 103 can be configured as follows.

As shown in FIG. 4 a, a first embodiment of the LCM comprises one liquidcrystal layer 1031. In this embodiment, a thickness, d, of the liquidcrystal layer 1031 should be configured large enough to achieve asufficient modulation depth of the phase retardation, which therebyrequires a larger drive voltage and a further reduction in frequencyresponse.

As shown in FIG. 4 b, a second embodiment of the LCM comprises two ormore liquid crystal layers 1032 stacked together to form a multi-layeredLCM 1033. This embodiment does not need a larger drive voltage, butrequires several liquid crystal layers.

The configuration of a third embodiment of the LCM is described as shownin FIG. 4 c. The LCM 1037 of this embodiment comprises a liquid crystallayer 1036 disposed between two mirrors 1034 and 1035. The position ofthe mirrors 1034 and 1035 is carefully configured so that the light canpass through the liquid crystal layer 1036 several times due to thereflection between the mirrors, which can thereby enhance the phaseretardation. According to this embodiment, the modulation depth actuallydepends on the number of times that the optical beam passes through theliquid crystal layer 1036.

Measured signal traces according to both single pass and multi-passmethods are shown in FIG. 6 a and FIG. 6 b, respectively. It isunderstood from the figures, the multi-pass method has a high efficiencyand a simple structure, which is demonstrated in the present invention.The triangular waveform shown in FIGS. 6 a and 6 b correspond to thedrive voltage of the LCM 1031 and 1037, respectively. In the case ofmulti-pass modulation, since the beam-folding mirrors 1034 and 1035 willforce the beam to go pass the liquid crystal layer 1036 a number oftimes, the aggregated modulation depth will increase accordingly inmultiples of that of the single pass case. As seen from FIG. 6 b, thesignal trace goes through many more oscillations in each sweep cycle ofthe LCM drive voltage. The multi-pass method will ensure that thedetected signal trace contains a higher level of spectral information,and consequently we can obtain a more accurate phase measurement fromthe signal trace. It is known that the retardation modulation providedby the LCM is not linearly proportional to the drive voltage. As aresult the signal traces obtained from our setup in both cases aresomewhat different from a pure sinusoidal waveform. However thisnon-linearity does not give any adverse effect on the final result whenthe differential phase measurement is conducted by first performingnormalization of the two signal traces concerned. Then this is followedby performing point-wise signal amplitude subtraction along thehorizontal time axis. The resultant signal tract will be a horizontalstraight line with its position on the vertical amplitude axisproportional to the differential phase. The non-linearity introduced bythe LCM will be automatically removed. Any small variations along thishorizontal line will signify noise from phase jitters. The finaldifferential phase value is obtained by taking an average of all thepoints along this horizontal line. The fact that we use the sameprocedures for all traces will ensure that no significant error will beintroduced to the final differential phase value.

Now referring to FIG. 2, a beam splitter 104, which may be a blank glassslide, is used to separate the beam of light into two parts. One part isfor a probe beam and the other, which is only 4% of input intensity, isfor a reference beam. The reference beam can be obtained by thereference unit 200 which comprises a polarizer 201 and a photodetector202. As shown in FIG. 2, the polarizer 201 is placed in front of thephotodetector 202 for picking up the reference beam. The polarizer 201is set at 45° relative to an optical axis of the p-polarization togenerate an interference between the p- and s-polarization components.The photodetector 202 is used to detect the interference and convert anoptical intensity variation of the interference to an electrical signal.Since the LCM 103 can permit phase retardation modulation of any formataccording to the present invention, it is possible to implementfull-field SPR phase imaging simply by monitoring the SPR sensor surfacein real-time using a photodetector 202 while the retardation phase iscontinuously increased in steps. According to the embodiment, thephotodetector 202 can be a photo-diode or a photo charge coupled device(CCD). Apart from providing the required reference retardation phase,the output from the photodetector 202 also offers important informationon the temperature characteristics of the LCM 103 which can later beused for providing a temperature compensation of the measured SPR phase.

The sensing unit 300 is used for receiving another portion, about 96%,of the beam of light to induce a phase retardation change between the p-and s-polarization components due to SPR associated with a sample. It isunderstood by those skilled in the art that any SPR coupling schemes,such as prism coupling, waveguide coupling, and grating coupling, may beapplied to the present invention. In the embodiment, a prism couplingscheme (prism/metal layer/sample) is preferably used and theconfiguration thereof is shown in FIGS. 7 and 8. In this configuration,the prism coupling scheme comprises a prism 301, a transducing layer 302coated on a surface of the prism to serve as a sensing surface, and asample flow chamber 304 associated with the prism 301 for allowing asample 303 flowing through the sensing surface. The prism 301 can bemade by transparent dielectric material such as plastic or glass inorder to enhance the momentum of light to match with the momentum ofSPW. In this embodiment, a dove prism made by BK7 glass is employed. Thetransducing layer 302 is commonly made by conducting material such asgold or silver. In this embodiment, a gold thin layer, about 48 nm, isemployed because of its good chemical resistance. The thickness of sucha layer is normally at the range of 20 nm to 80 nm depending onapplications and material selection. The sample 303 is normally used inaqueous form. Glycerin and water mixtures in the concentration of weightpercentage from 0% to 16% were used in this embodiment. The sample flowchamber 304 is designed for permitting the sample 303 flowing in and outthe chamber 304 while contacting the sensing surface. Since only thep-polarization component is affected by the SPR effect, and thes-polarization component is kept intact, a phase retardation changebetween the p- and s-polarization components can be introduced by theSPR according to the concentration of weight percentage of the sample.

As shown in FIG. 8, the probe unit 400 is used to provide a probe signalindicating the phase retardation change by receiving the light afterSPR. Similar to the reference unit 200, the probe unit 400 comprises apolarizer 401 and a photodetector 402. The polarizer 401 is oriented sothat the p- and s-polarized components can interfere with each another.Then the photodetector 402 is used to capture the optical intensityvariation. Depending on the designed applications, the photodetector 402can be a photo-diode or a photo charge coupled device (CCD). In theembodiment, a silicon photo-diode is employed as the photodetector 402.

As shown in FIG. 8, the detection unit 500, which is connected betweenthe reference unit 200 and the probe unit 400, comprises a phase meter501 and a microprocessor 502. The phase meter 501 is employed to measurea differential phase between the reference signal and the probe signalso that the microprocessor 502 is able to determine biological,biochemical, or chemical characteristics of the sample in accordancewith the differential phase.

Typical signal waveform collected from the probe unit and reference unitare shown in FIG. 9. After processed by the phase extraction analysisprogram, the differential phase value between the probe and referencesignals is obtained from the signal waveforms. To demonstrate theperformance of the present invention, the concentration measurement ofglycerin and water mixtures from 0% to 16% in weight percentage with thecorresponding refractive index unit (RIU) ranging from 1.3330 to 1.3521was conducted. The experimental differential phase variation versusglycerin concentration is shown in FIG. 10. According to this plot, thephase response was found to be 1.7×10⁻⁶ RIU/degree in the mostsensitivity region and the system stability was also measured to be lessthan 0.15° within an hour as shown in FIG. 11. Using this stabilityvalue as the phase resolution, the calculated system sensitivity wastherefore 2.6×10⁻⁷ RIU.

The SPR sensor according to the present invention generates a requiredsignal in the time domain through modulating a phase retardance of abeam using a LCM, hence leading to much reduced system hardwarecomplexity. More importantly, the removal of angular measurement alsoenables full field imaging of the SPR sensor surface, which is apre-requisite for two-dimensional sensor arrays. In addition, signal tonoise ratios may be easily enhanced according to needs through signalaveraging. Digital data processing such as narrowband filtering may alsobe applied to the invention to improve the measurement precision.

While we have hereinbefore described embodiments of this invention, itis understood that our basic constructions can be altered to provideother embodiments which utilize the processes and compositions of thisinvention. Consequently, it will be appreciated that the scope of thisinvention is to be defined by the claims appended hereto rather than bythe specific embodiments which have been presented hereinbefore by wayof examples.

1. An optical sensing device comprising: a source unit, comprising: asource emitting a beam of light containing p-polarization ands-polarization components, and a phase modulator modulating a phaseretardation between the p-polarization and s-polarization components; areference unit, configured to receive a first part of the light, therebyproviding a reference signal indicating the modulated phase retardation;a SPR sensing unit, configured to receive a second part of the light,thereby inducing a phase retardation change between the p-polarizationand s-polarization components due to SPR associated with a sample; aprobe unit, configured to receive the light passing the SPR sensing unitthereby providing a probe signal indicating the phase retardation changeinduced by the SPR sensing unit; and a detection unit, coupled betweenthe reference unit and the probe unit to detect biological, biochemical,or chemical characteristics of the sample by comparing the referencesignal with the probe signal.
 2. The optical sensing device according toclaim 1, wherein the phase modulator is a liquid crystal modulator andthe phase retardation is continuously modulated by the liquid crystalmodulator.
 3. The optical sensing device according to claim 2, whereinthe liquid crystal modulator comprises one liquid crystal layer.
 4. Theoptical sensing device according to claim 2, wherein the liquid crystalmodulator comprises two or more liquid crystal layers stacked together.5. The optical sensing device according to claim 2, wherein the liquidcrystal modulator comprises two mirrors and a liquid crystal layerdisposed between the mirrors.
 6. The optical sensing device according toclaim 1, wherein the source of the source unit is a gas laser, a solidstate laser, a laser diode, a light emitting diode, or a broad bandwhite light source.
 7. The optical sensing device according to claim 1,wherein the source unit further comprises: a polarizer for selecting anintensity ratio between the p-polarization and s-polarization componentsof the light emitted from the source; and a beam splitter for separatingthe modulated light into the first part and the second part.
 8. Theoptical sensing device according to claim 1, wherein the reference unitcomprises: a polarizer employed to form an interference between thep-polarization and s-polarization components of the first part of light;and a photodetector detecting the interference and converting an opticalintensity variation of the interference to the reference signalindicating the phase retardation.
 9. The optical sensing deviceaccording to claim 8, wherein the photodetector is a photo-diode or aphoto charge coupled device.
 10. The optical sensing device according toclaim 1, wherein the sensing unit comprises: a prism; a transducinglayer coated on a surface of the prism to serve as a sensing surface;and a sample flow chamber associated with the prism allowing the sampleflowing through the sensing surface.
 11. The optical sensing deviceaccording to claim 1, wherein the probe unit comprises: a polarizeremployed to form an interference between the p-polarization ands-polarization components of the second part of light; and aphotodetector detecting the interference and converting an opticalintensity variation of the interference to the probe signal indicatingthe phase retardation change induced by the SPR sensing unit.
 12. Theoptical sensing device according to claim 11, wherein the photodetectoris a photo-diode or a photo charge coupled device.
 13. The opticalsensing device according to claim 1, wherein the detection unitcomprises: a phase meter for measuring a differential phase between thereference signal and the probe signal; and a microprocessor fordetermining the biological, biochemical, or chemical characteristics ofthe sample in accordance with the differential phase.
 14. A method ofdetecting biological, biochemical, or chemical characteristics of asample comprising: transmitting a beam of light containingp-polarization and s-polarization components; modulating a phaseretardation between the p-polarization and s-polarization components;providing a reference signal indicating the phase retardation byreceiving a first part of the light; receiving a second part of thelight to induce a phase retardation change between the p-polarizationand s-polarization components due to SPR associated with a sample;providing a probe signal indicating the phase retardation change byreceiving the light after SPR associated with the sample; and detectingbiological, biochemical, or chemical characteristics of the sample bycomparing the probe signal with the reference signal.
 15. The methodaccording to claim 14, wherein the phase retardation is continuouslymodulated by a liquid crystal modulator through varying a drive voltageof the liquid crystal modulator.
 16. The method according to claim 15,wherein the drive voltage of the liquid crystal modulator is varied from0v to 13v and a frequency of the liquid modulator is set to be 10 Hz.17. The method according to claim 15, wherein the liquid crystalmodulator comprises one liquid crystal layer, and a thickness of theliquid crystal layer is designed so that a modulation depth of theliquid crystal modulator is greater than 2π.
 18. The method according toclaim 15, wherein the liquid crystal modulator comprises two or moreliquid crystal layers stacked together to form a multi-layeredmodulator, and the number of the liquid crystal layers and a thicknessof each liquid crystal layer are designed so that a modulation depth ofthe liquid crystal modulator is greater than 2π.
 19. The methodaccording to claim 15, wherein the liquid crystal modulator comprisestwo mirrors and a liquid crystal layer disposed between the mirrors, anda thickness of the liquid crystal layer and a position of the mirrorsand liquid crystal layer are designed so that a modulation depth of theliquid crystal modulator is greater than 2π.
 20. The method according toclaim 14, wherein said transmitting a beam of light further comprisesselecting an intensity ratio between the p-polarization ands-polarization components.
 21. The method according to claim 14, whereinsaid providing a reference signal comprises: generating an interferencebetween the p-polarization and s-polarization components of the firstpart of light by means of a polarizer; and converting an opticalintensity variation of the interference to the reference signal.
 22. Themethod according to claim 21, wherein the reference signal is used tomonitor an effect of temperature variation.
 23. The method according toclaim 14, wherein said receiving a second part of light to induce aphase retardation change comprises: allowing the sample flowing througha chamber of an SPR sensor; and subjecting the second part of the lightto passing through the SPR sensor so that the phase retardation betweenthe p-polarization and s-polarization components of the second part oflight is changed due to the SPR at the SPR sensor.
 24. The methodaccording to claim 14, wherein said providing a probe signal comprises:generating an interference between the p-polarization and s-polarizationcomponents of the light after SPR by means of a polarizer; andconverting an optical intensity variation of the interference to theprobe signal.
 25. The method according to claim 14, wherein saiddetecting the biological, biochemical, or chemical characteristics ofthe sample comprises: measuring a differential phase between thereference signal and the probe signal; and determining biological,biochemical, or chemical characteristics of the sample in accordancewith the differential phase.